Recently, a transmission method is demanded by which a large amount of data is optically transmitted through an optical network. Multi-carrier modulation methods such as, for example, a discrete multi-tone (DMT) modulation method are known as transmission methods in which an optical network is used. The DMT modulation method is one of multi-carrier transmission technologies based on the orthogonal frequency division multiplexing (OFDM). In the DMT modulation method, data is allocated to a plurality of subcarriers (SCs) with different frequencies, after which multi-level modulation is performed on data allocated to each SC and the data is transmitted at high speed as a DMT signal.
An optical transmission apparatus in the DMT modulation method negotiates with a distant optical transmission apparatus with a probe signal at the time of, for example, a system startup. The optical transmission apparatus acquires a received characteristic according to a negotiation result, and sets the received characteristic as a transmission characteristic. According to the transmission characteristic setting, the optical transmission apparatus executes bit and power loading to determine the amount of modulation-level (number of bits) and signal power allocation for each SC. The device characteristics of an optical transmitter and optical receiver included in the optical transmission apparatus largely affect transmission characteristics.
Japanese Laid-open Patent Publication No. 2004-112781 is an example of related art.
However, after the optical transmission apparatus in the DMT modulation method has set the transmission characteristic according to the negotiation result, the device characteristics of the optical transmitter and optical receiver vary due to changes in the ambient temperature, changes in the power supply voltage, and other factors. When the power supply voltage drops, the frequency characteristic of an amplifier in the optical receiver, for example, is degraded, lowering the gain and narrowing the bandwidth. The frequency characteristic of the amplifier in the optical receiver is also degraded when the ambient temperature is raised, in which case to the gain is lowered and the bandwidth is narrowed. Particularly, when the device characteristics of the optical transmitter and optical receiver in the optical transmission apparatus are degraded, these device characteristics deviate from the transmission characteristic setting. As a result, it is difficult to maintain the transmission characteristics using allocated setting based on the initial condition. Probably it may be cause a transmission error.
In addition, if an optical transmission path between the optical transmission apparatus and the distant optical transmission apparatus is long, the characteristic of the optical transmission path and the device characteristics of a relay and other devices placed at intermediate points of the optical transmission path affect the transmission characteristic setting. Particularly, if optical multi-pass interference (MPI) occurs in the transmission path, the transmission characteristic of a DMT signal in a low-frequency bandwidth is largely degraded. As a result, a deviation from the transmission characteristic setting becomes noticeable. This makes it is difficult to maintain the transmission characteristics using allocated setting based on the initial condition. Probably it may be cause a transmission error.
FIGS. 16A and 16B illustrate examples of a transmission characteristic and the amount of allocation before and after the transmission characteristic is degraded. An optical transmission apparatus sets the amount M1 of modulation level and signal power allocation for each SC according to a transmission characteristic T1, which has not been degraded, illustrated in FIG. 16A. If the transmission characteristic is degraded during an operation, the transmission characteristic T1 is lowered as indicated by a transmission characteristic T2 illustrated in FIG. 16B, in which case it is hard to obtain the amount of modulation level and the set amount M1 of modulation level and signal power allocation for each SC, probably causing an error in the transmission of a DMT signal. That is, if a characteristic is degraded during an operation, a DMT signal may cause a transmission error.